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Interaction with two microplastic types by an adult freshwater mussel produces differences in ingestion and egestion patterns
Summary
Freshwater mussels absorb tiny plastic beads and plastic fibers differently, beads spread quickly into organs like the gills and reproductive tissue but got stuck there, while fibers took longer to show up but were flushed out more easily over a week. This matters because these mussels are eaten by wildlife and are also used as indicators of water quality, so understanding how different plastic shapes build up in their tissues helps scientists predict how microplastics might move through the food chain and eventually reach the foods we eat, like fish and shellfish.
Microplastic (MP) ubiquity has led to their detection in many filter feeding organisms in aquatic environments. Interaction of MPs with filter feeding marine bivalves is known to vary with particle morphology, yet temporal dynamics, organ-specific retention and egestion patterns remain poorly characterized in freshwater bivalve species. We exposed the adult freshwater unionid mussel Megalonaias nervosa (washboard) to 150,000 MP/mL of 6 μm polystyrene (PS) spheres and 61 μm polyester (PES) fibers to determine ingestion and egestion patterns, and how these may differ between MPs. Accumulation dynamics differed markedly between particle types; PS sphere concentrations in mussel organs remained stable throughout the MP exposure period, while PES fibers increased logarithmically as time progressed. Presence of PS spheres was associated with each organ examined after only one day of exposure, while PES fibers were not detected in the gills and gonad until the third day of exposure. Counts of PS spheres in organs followed a ranking of digestive gland > gonad > gills > hemolymph > foot, while PES fiber counts ranked as digestive gland > gills > foot > gonad > hemolymph. The digestive gland was the primary site of MP accumulation, with counts being at least 60 times greater than any other organ examined. After 48 hr, M. nervosa depurated PES fibers more efficiently than PS spheres as indicated by the continuous egestion of PES, but not PS, throughout the 7-day MP exposure. Further, the feces and pseudofeces produced by M. nervosa when exposed to PES fibers was significantly higher in organic matter content. Adult M. nervosa in this study exhibited significantly different ingestion and egestion patterns according to different MP types. This suggests that MP characteristics influence availability and translocation into non-digestive organs, highlighting the importance of studying multiple particle types when considering the accumulation and depuration of MPs in filter feeding organisms.